Modem
A modem (portmanteau of modulator-demodulator) is a hardware device that converts data into a format suitable for a transmission medium so that it can be transmitted from computer to computer (historically over telephone wires). A modem modulates one or more carrier wave signals to encode digital information for transmission and demodulates signals to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded reliably to reproduce the original digital data. Modems can be used with almost any means of transmitting analog signals from light-emitting diodes to radio. A common type of modem is one that turns the digital data of a computer into modulated electrical signal for transmission over telephone lines and demodulated by another modem at the receiver side to recover the digital data. Modems are generally classified by the maximum amount of data they can send in a given unit of time, usually expressed in bits per second (symbol bit(s), sometimes abbreviated "bps") or rarely in bytes per second (symbol B(s)). Modems can also be classified by their symbol rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency-shift keying with two possible frequencies, corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols (two bits per symbol), transmitted 1,200 bits by sending 600 symbols per second (600 baud) using phase-shift keying. Dial-up modem History News wire services in the 1920s used multiplex devices that satisfied the definition of a modem. However, the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems. Modems grew out of the need to connect teleprinters over ordinary phone lines instead of the more expensive leased lines which had previously been used for current loop–based teleprinters and automated telegraphs. In 1941, the Allies developed a voice encryption system called SIGSALY which used a vocoder to digitize speech, then encrypted the speech with one-time pad and encoded the digital data as tones using frequency shift keying. Mass-produced modems in the United States began as part of the SAGE air-defense system in 1958 (the year the word modem was first used ), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the United States and Canada. SAGE modems were described by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems. The 201A and 201B Data-Phones were synchronous modems using two-bit-per-baud phase-shift keying (PSK). The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels each running on their own set of two wires. The famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, and the IBM 2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems. Acoustic couplers For many years, the Bell System (AT&T) maintained a monopoly on the use of its phone lines and what devices could be connected to them. However, the FCC's seminal Carterfone Decision of 1968, the FCC concluded that electronic devices could be connected to the telephone system as long as they used an acoustic coupler. Since most handsets were supplied by Western Electric and thus of a standard design, acoustic couplers were relatively easy to build. Acoustically coupled Bell 103A-compatible 300 bit/s modems were common during the 1970s. Well-known models included the Novation CAT and the Anderson-Jacobson, the latter spun off from an in-house project at Stanford Research Institute (now SRI International). An even lower-cost option was the Pennywhistle modem, designed to be built using parts from electronics scrap and surplus stores. In December 1972, Vadic introduced the VA3400, notable for full-duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different frequency bands for transmit and receive. In November 1976, AT&T introduced the 212A modem to compete with Vadic. It was similar in design, but used the lower frequency set for transmission. One could also use the 212A with a 103A modem at 300 bit/s. According to Vadic, the change in frequency assignments made the 212 intentionally incompatible with acoustic coupling, thereby locking out many potential modem manufacturers. In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to computer center operators that supported Vadic's 1,200-bit/s mode, AT&T's 212A mode, and 103A operation. Carterfone and direct connection The Hush-a-Phone decision applied only to mechanical connections, but the Carterfone decision of 1968, led to the FCC introducing a rule setting stringent AT&T -designed tests for electronically coupling a device to the phone lines. This opened the door to direct-connect modems that plugged directly into the phone line rather than via a handset. However, the cost of passing the tests was considerable, and acoustically coupled modems remained common into the early 1980s. The rapidly falling prices of electronics in the late 1970s led to an increasing number of direct-connect models around 1980. In spite of being directly connected, these modems were generally operated like their earlier acoustic versions – dialing and other phone-control operations were completed by hand, using an attached handset. A small number of modems added the ability to automatically answer incoming calls, or automatically place an outgoing call to a single number, but even these limited features were relatively rare or limited to special models in a lineup. When more flexible solutions were needed, third party "dialers" were used to automate calling, normally using a separate serial port to communicate with the dialer, which would then control the modem through a private electrical connection. The introduction of microcomputer systems with internal expansion slots made the first software-controllable modems common. Slot connections gave the computer complete access to the modem's memory or input/output (I/O) channels, which allowed software to send commands to the modem, not just data. This led to a series of popular modems for the S-100 bus and Apple II computers that could directly dial the phone, answer incoming calls, and hang up the phone, the basic requirements of a bulletin board system (BBS). The seminal CBBS was created on an S-100 machine with a Hayes internal modem, and a number of similar systems followed. The Smartmodem and the rise of BBSs The next major advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300 bit/s direct-connect modem, but it was attached to a small microcontroller that watched the data stream for certain character strings representing commands. This allowed both data and commands to be sent through a single serial port. The now-standard Hayes command set included instructions for picking up and hanging up the phone, dialing numbers, and answering calls, among others. This was similar to the commands offered by the internal modems, but unlike them, the Smartmodem could be connected to any computer with an RS-232 port, which was practically every microcomputer built. The introduction of the Smartmodem made communications much simpler and more easily accessed. This provided a growing market for other vendors, who licensed the Hayes patents and competed on price or by adding features. Through the 1980s, a number of new higher-speed modems, first 1,200 and then 2,400 bit/s, greatly improved the responsiveness of the online systems, and made file transfer practical. This led to rapid growth of online services with their large file libraries, which in turn gave more reason to own a modem. The rapid update of modems led to a similar rapid increase in BBS use, which was helped by the fact that BBSs could control the modem simply by sending strings, rather than talking to a device driver that was different for every direct-connect modem. 1200 and 2400 bit/s The 300 bit/s modems used audio frequency-shift keying to send data. In this system, the stream of 1s and 0s in computer data is translated into sounds which can be easily sent on the phone lines. In the Bell 103 system, the originating modem sends 0s by playing a 1,070 Hz tone, and 1s at 1,270 Hz, with the answering modem transmitting its 0s on 2,025 Hz and 1s on 2,225 Hz. These frequencies were chosen carefully; they are in the range that suffers minimum distortion on the phone system and are not harmonics of each other. In the 1,200 bit/s and faster systems, phase-shift keying was used. In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase. Voiceband modems generally remained at 300 and 1,200 bit/s (V.21 and V.22) into the mid-1980s. A V.22bis 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212 signaling was introduced in the U.S., and a slightly different one in Europe. The limited available frequency range meant the symbol rate of 1,200 bit/s modems was still only 600 baud (symbols per second). The bit rate increases were achieved by defining four or eight distinct symbols, which allowed the encoding of two or three bits per symbol instead of only 1. The use of smaller shifts had the drawback of making each symbol more vulnerable to interference, but improvements in phone line quality at the same time helped compensate for this. By the late 1980s, most modems could support all of these standards and 2,400-bit/s operation was becoming common. Proprietary standards Many other standards were also introduced for special purposes, commonly using a high-speed channel for receiving, and a lower-speed channel for sending. One typical example was used in the French Minitel system, in which the user's terminals spent the majority of their time receiving information. The modem in the Minitel terminal thus operated at 1,200 bit/s for reception, and 75 bit/s for sending commands back to the servers. Three U.S. companies became famous for high-speed versions of the same concept. Telebit introduced its Trailblazer modem in 1984, which used a large number of 36 bit/s channels to send data one-way at rates up to 18,432 bit/s. A single additional channel in the reverse direction allowed the two modems to communicate how much data was waiting at either end of the link, and the modems could change direction on the fly. The Trailblazer modems also supported a feature that allowed them to spoof the UUCP g'' protocol, commonly used on Unix systems to send e-mail, and thereby speed UUCP up by a tremendous amount. Trailblazers thus became extremely common on Unix systems, and maintained their dominance in this market well into the 1990s. USRobotics (USR) introduced a similar system, known as ''HST, although this supplied only 9,600 bit/s (in early versions at least) and provided for a larger backchannel. Rather than offer spoofing, USR instead created a large market among FidoNet users by offering its modems to BBS sysops at a much lower price, resulting in sales to end users who wanted faster file transfers. Hayes was forced to compete, and introduced its own 9,600 bit/s standard, Express 96 (also known as Ping-Pong), which was generally similar to Telebit's PEP. Hayes, however, offered neither protocol spoofing nor sysop discounts, and its high-speed modems remained rare. A common feature of these high-speed modems was the concept of fallback, or speed hunting, allowing them to communicate with less-capable modems. During the call initiation, the modem would transmit a series of signals and wait for the remote modem to respond. They would start at high speeds and get progressively slower until there was a response. Thus, two USR modems would be able to connect at 9,600 bit/s, but, when a user with a 2,400 bit/s modem called in, the USR would fall back to the common 2,400 bit/s speed. This would also happen if a V.32 modem and a HST modem were connected. Because they used a different standard at 9,600 bit/s, they would fall back to their highest commonly supported standard at 2,400 bit/s. The same applies to V.32bis and 14,400 bit/s HST modem, which would still be able to communicate with each other at 2,400 bit/s. Echo cancellation, 9600 and 14,400 Sportster 14,400 Fax modem (1994)]] Echo cancellation was the next major advance in modem design. Local telephone lines use the same wires to send and receive data, which results in a small amount of the outgoing signal being reflected back. This is useful for people talking on the phone, as it provides a signal to the speaker that their voice is making it through the system. However, this reflected signal causes problems for the modem, which is unable to distinguish between a signal from the remote modem and the echo of its own signal. This was why earlier modems split the signal frequencies into "answer" and "originate"; the modem could then ignore any signals in the frequency range it was using for transmission. Even with improvements to the phone system allowing higher speeds, this splitting of available phone signal bandwidth still imposed a half-speed limit on modems. Echo cancellation eliminated this problem. During the call setup and negotiation period, both modems send a series of unique tones and then listen for them to return through the phone system. They measure the total delay time and then set up a local delay loop to the same time. Once the connection is completed, they send their signals into the phone lines as normal, but also into the delay, which is inverted. The signal returning through the echo meets the inverted version coming from the delay line, and cancels out the echo. This allowed both modems to use the full spectrum available, doubling the speed. Additional improvements were introduced via the quadrature amplitude modulation (QAM) encoding system. Previous systems using phase shift keying (PSK) encoded two bits (or sometimes three) per symbol by slightly delaying or advancing the signal's phase relative to a set carrier tone. QAM used a combination of phase shift and amplitude to encode four bits per symbol. Transmitting at 1,200 baud produced the 4,800 bit/s V.27ter standard, the same working at a base rate of 2,400 baud produced the 9,600 bit/s V.32. The carrier frequency was 1,650 Hz in both systems. For many years, most engineers considered this rate to be the limit of data communications over telephone networks. The introduction of these higher-speed systems also led to the digital fax machine during the 1980s. Digital faxes are simply an image format sent over a high-speed (commonly 14.4 kbit/s) modem. Software running on the host computer can convert any image into fax format, which can then be sent using the modem. Such software was at one time an add-on, but has since become largely universal. Breaking the 9.6 kbit/s barrier The first 9,600 bit/s modem was developed in 1968, and sold for more than $20,000, but had high error rates. In 1980, Gottfried Ungerboeck from IBM Zurich Research Laboratory applied channel coding techniques to search for new ways to increase the speed of modems. His results were astonishing but only conveyed to a few colleagues. In 1982, he agreed to publish what is now a landmark paper in the theory of information coding. By applying parity check coding to the bits in each symbol, and mapping the encoded bits into a two-dimensional diamond pattern, Ungerboeck showed that it was possible to increase the speed by a factor of two with the same error rate. The new technique was called mapping by set partitions, now known as trellis modulation. Error correcting codes, which encode code words (sets of bits) in such a way that they are far from each other, so that in case of error they are still closest to the original word (and not confused with another) can be thought of as analogous to sphere packing or packing pennies on a surface: the further two bit sequences are from one another, the easier it is to correct minor errors. Dave Forney introduced the trellis diagram in a landmark 1973 paper that popularized the Viterbi algorithm. Mark Anderson. "David Forney: The Man Who Launched a Million Modems". 2016. Practically all modems operating faster than 9600 bit/s decode trellis-modulated data using the Viterbi algorithm. V.32 modems operating at 9600 bit/s were expensive and were only starting to enter the market in the early 1990s when V.32bis was standardized. Rockwell International's chip division developed a new driver chip set incorporating the standard and aggressively priced it. Supra, Inc. arranged a short-term exclusivity arrangement with Rockwell, and developed the SupraFAXModem 14400 based on it. Introduced in January 1992 at $399 (or less), it was half the price of the slower V.32 modems already on the market. This led to a price war, and by the end of the year V.32 was dead, never having been really established, and V.32bis modems were widely available for $250. V.32bis was so successful that the older high-speed standards had little to recommend them. USR fought back with a 16,800 bit/s version of HST, while AT&T introduced a one-off 19,200 bit/s method they referred to as V.32ter, but neither non-standard modem sold well. V.34/28.8 kbit/s and 33.6 kbit/s card]] for notebooks]] Any interest in these proprietary improvements was destroyed during the lengthy introduction of the 28,800 bit/s V.34 standard. While waiting, several companies decided to release hardware and introduced modems they referred to as V.FAST. In order to guarantee compatibility with V.34 modems once the standard was ratified (1994), the manufacturers were forced to use more flexible parts, generally a DSP and microcontroller, as opposed to purpose-designed ASIC modem chips. The ITU standard V.34 represents the culmination of the joint efforts. It employs the most powerful coding techniques including channel encoding and shape encoding. From the mere four bits per symbol (9.6 kbit/s), the new standards used the functional equivalent of 6 to 10 bits per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 28.8, and 33.6 kbit/s modems. This rate is near the theoretical Shannon limit. When calculated, the Shannon capacity of a narrowband line is \text{bandwidth} \times \log_2 (1 + P_u/P_n) , with P_u/P_n the (linear) signal-to-noise ratio. Narrowband phone lines have a bandwidth of 3,000 Hz so using P_u/P_n=1000 (SNR = 30 dB), the capacity is approximately 30 kbit/s. Using digital lines and PCM (V.90/92) During the late 1990s, Rockwell-Lucent and USRobotics introduced competing technologies based upon the digital transmission used in telephony networks. The standard digital transmission in modern networks is 64 kbit/s but some networks use a part of the bandwidth for remote office signaling (e.g. to hang up the phone), limiting the effective rate to 56 kbit/s DS0. This new technology was adopted into ITU standards V.90 and is common in modern computers. The 56 kbit/s rate is only possible from the central office to the user site (downlink). In the United States, government regulation limits the maximum power output, resulting in a maximum data rate of 53.3 kbit/s. The uplink (from the user to the central office) still uses V.34 technology at 33.6 kbit/s. USRobotics began work on the technology first, calling theirs X2 because 56k was twice the speed of 28k modems. USRobotics held a 40-percent share of the retail modem market, and Rockwell International held an 80-percent share of the modem chipset market. Concerned with being shut out of the market, Rockwell began work on a rival 56k technology and joined with Lucent and Motorola on what it called K56Flex or Flex. Both technologies reached the market around February 1997; although problems with K56Flex modems were noted in product reviews through July, within six months they worked equally well with variations dependent on local connection characteristics. The retail price of the 56K modems was about US$200, compared to $100 for 33K modems. Separate equipment was required by Internet service providers (ISPs) to support the incompatible technologies, with costs varying depending on whether their current equipment could be upgraded. About half of all ISPs offered 56K support by October 1997. Consumer sales were relatively low, which USRobotics and Rockwell attributed to conflicting standards. The International Telecommunication Union (ITU) announced the draft of a new 56 kbit/s standard, V.90, in February 1998, with strong industry support. Incompatible with either existing standard, it was an amalgam of both which was designed to allow both types of modem to be converted to it by a firmware upgrade. This V.90 standard was approved in September 1998, and widely adopted by ISPs and consumers. Later in V.92, the digital PCM technique was applied to increase the upload speed to a maximum of 48 kbit/s, but at the expense of download rates. A 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s due to echo on the telephone line. To avoid this problem, V.92 modems offer the option to turn off the digital upstream and instead use a 33.6 kbit/s analog connection, in order to maintain a high digital downstream of 50 kbit/s or higher. V.92 also adds two other features. The first is the ability for users who have call waiting to put their dial-up Internet connection on hold for extended periods of time while they answer a call. The second feature is the ability to quickly connect to one's ISP. This is achieved by remembering the analog and digital characteristics of the telephone line, and using this saved information when reconnecting. Using compression to exceed 56 kbit/s V.42, V.42bis and V.44 standards allow the modem to transmit data faster than its basic rate would imply. For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3 × 6 = 320 kbit/s using pure text. However, the compression ratio tends to vary due to noise on the line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video). At some points the modem will be sending compressed files at approximately 50 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between. - Wolfgang Henke. In such situations, a small amount of memory in the modem, a buffer, is used to hold the data while it is being compressed and sent across the phone line, but in order to prevent overflow of the buffer, it sometimes becomes necessary to tell the computer to pause the datastream. This is accomplished through hardware flow control using extra lines on the modem–computer connection. The computer is then set to supply the modem at some higher rate, such as 320 kbit/s, and the modem will tell the computer when to start or stop sending data. Compression by the ISP As telephone-based 56k modems began losing popularity, some Internet service providers such as Netzero/Juno, Netscape, and others started using pre-compression to increase the throughput and maintain their customer base. The server-side compression operates much more efficiently than the on-the-fly compression done by modems because these compression techniques are application-specific (JPEG, text, EXE, etc.). The website text, images, and Flash executables are compacted to approximately 4%, 12%, and 30%, respectively. The drawback of this approach is a loss in quality, which causes image content to become pixelated and smeared. ISPs employing this approach often advertise it as "accelerated dial-up". These accelerated downloads are now integrated into the Opera and Amazon Silk web browsers, using their own server-side text and image compression. Softmodem A Winmodem or SoftModem is a stripped-down modem that replaces tasks traditionally handled in hardware with software. In this case the modem is a simple interface and codec to code and decode the digital signal. Softmodems are cheaper than traditional modems because they have fewer hardware components. However, the software interpreting the modem tones to be sent to the softmodem uses some system resources. For online gaming, this can be a real concern . Another problem is the lack of cross-platform compatibility, meaning that non-Windows operating systems (such as Linux) often do not have an equivalent driver to operate the modem. List of dial-up speeds These values are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines). For a complete list see the companion article list of device bandwidths. A baud is one symbol per second; each symbol may encode one or more data bits. Popularity A 1994 Software Publishers Association found that although 60% of computers in US households had a modem, only 7% of households went online. A CEA study in 2006 found that dial-up Internet access is declining in the U.S. In 2000, dial-up Internet connections accounted for 74% of all U.S. residential Internet connections. The United States demographic pattern for dial-up modem users per capita has been more or less mirrored in Canada and Australia for the past 20 years. Dial-up modem use in the U.S. had dropped to 60% by 2003, and in 2006, stood at 36%. Voiceband modems were once the most popular means of Internet access in the U.S., but with the advent of new ways of accessing the Internet, the traditional 56K modem is losing popularity. The dial-up modem is still widely used by customers in rural areas, where DSL, cable, satellite, or fiber optic service is not available, or they are unwilling to pay what these companies charge. In its 2012 annual report, AOL showed it still collected around US$700 million in fees from dial-up users: about three million people. Broadband ]] ADSL (asymmetric digital subscriber line) modems, a more recent development, are not limited to the telephone's voiceband audio frequencies. Standard twisted-pair telephone cable can, for short distances, carry signals with much higher frequencies than the cable's maximum frequency rating. ADSL broadband takes advantage of this capability. However, ADSL's performance gradually declines as the telephone cable's length increases. This limits ADSL broadband service to subscribers within a relatively short distance of the telephone exchange. Cable modems use infrastructure originally intended to carry television signals and therefore designed from the outset to carry higher frequencies. A single cable can carry radio and television signals at the same time as broadband internet service without interference. Newer types of broadband modems are also available, including satellite modems and power line modems. Most consumers did not know about networking and routers when broadband became available. However, many people knew that a modem connected a computer to the Internet over a telephone line. To take advantage of consumers' familiarity with modems, companies called these devices broadband modems rather than using less familiar terms such as adapter, interface, transceiver, or bridge. In fact, broadband modems fit the definition of modem because they use complex waveforms to carry digital data. They use more advanced technology than dial-up modems: typically they can modulate and demodulate hundreds of channels simultaneously or use much wider channels than dial-up modems. Radio Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as do most other wireless services today. Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required. Such systems are an important part of the PSTN, and are also in common use for high-speed computer network links to outlying areas where fibre is not economical. Even where a cable is installed, it is often possible to get better performance or make other parts of the system simpler by using radio frequencies and modulation techniques through a cable. Coaxial cable has a very large bandwidth, but signal attenuation becomes a major problem at high data rates if a baseband digital signal is used. By using a modem, a much larger amount of digital data can be transmitted through a single wire. Digital cable television and cable Internet services use radio frequency modems to provide the increasing bandwidth needs of modern households. Using a modem also allows for frequency-division multiple access to be used, making full-duplex digital communication with many users possible using a single wire. Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are often referred to as transparent or smart. They transmit information that is modulated onto a carrier frequency to allow many simultaneous wireless communication links to work simultaneously on different frequencies. Transparent modems operate in a manner similar to their phone line modem cousins. Typically, they were half duplex, meaning that they could not send and receive data at the same time. Typically, transparent modems are polled in a round robin manner to collect small amounts of data from scattered locations that do not have easy access to wired infrastructure. Transparent modems are most commonly used by utility companies for data collection. Smart modems come with media access controllers inside, which prevents random data from colliding and resends data that is not correctly received. Smart modems typically require more bandwidth than transparent modems, and typically achieve higher data rates. The IEEE 802.11 standard defines a short range modulation scheme that is used on a large scale throughout the world. WiFi and WiMax The WiFi and WiMax standards use wireless mobile broadband modems operating at microwave frequencies. Mobile broadband HSPA+ Evolution-Data Optimized (EVDO) USB wireless modem from Movistar Colombia]] Modems which use a mobile telephone system (GPRS, UMTS, HSPA, EVDO, WiMax, etc.), are known as mobile broadband modems (sometimes also called wireless modems). Wireless modems can be embedded inside a laptop or appliance, or be external to it. External wireless modems are connect cards, USB modems for mobile broadband and cellular routers. A connect card is a PC Card or ExpressCard which slides into a PCMCIA/PC card/ExpressCard slot on a computer. USB wireless modems use a USB port on the laptop instead of a PC card or ExpressCard slot. A USB modem used for mobile broadband Internet is also sometimes referred to as a dongle. A cellular router may have an external datacard (AirCard) that slides into it. Most cellular routers do allow such datacards or USB modems. Cellular routers may not be modems by definition, but they contain modems or allow modems to be slid into them. The difference between a cellular router and a wireless modem is that a cellular router normally allows multiple people to connect to it (since it can route data or support multi-point to multi-point connections), while a modem is designed for one connection. Most of GSM wireless modems come with an integrated SIM cardholder (i.e., Huawei E220, Sierra 881, etc.) and some models are also provided with a microSD memory slot and/or jack for additional external antenna such as Huawei E1762 and Sierra Wireless Compass 885. The CDMA (EVDO) versions do not use R-UIM cards, but use Electronic Serial Number (ESN) instead. The cost of using a wireless modem varies from country to country. Some carriers implement flat rate plans for unlimited data transfers. Some have caps (or maximum limits) on the amount of data that can be transferred per month. Other countries have plans that charge a fixed rate per data transferred—per megabyte or even kilobyte of data downloaded; this tends to add up quickly in today's content-filled world, which is why many people are pushing for flat data rates. The faster data rates of the newest wireless modem technologies (UMTS, HSPA, EVDO, WiMax) are also considered to be broadband wireless modems and compete with other broadband modems below. Until the end of April 2011, worldwide shipments of USB modems surpassed embedded 3G and 4G modules by 3:1 because USB modems can be easily discarded, but embedded modems could start to gain popularity as tablet sales grow and as the incremental cost of the modems shrinks, so by 2016, the ratio may change to 1:1. Like mobile phones, mobile broadband modems can be SIM locked to a particular network provider. Unlocking a modem is achieved the same way as unlocking a phone, by using an 'unlock code'. Residential gateways Some devices referred to as "broadband modems" are residential gateways, integrating the functions of a modem, network address translation (NAT) router, Ethernet switch, WiFi access point, DHCP server, firewall, among others. Some residential gateway offer a so-called "bridged mode", which disables the built-in routing function and makes the device function similarly to a plain modem. This bridged mode is separate from RFC 1483 bridging. DC Powerline DC-BUS powerline modem provide communication over noisy power lines at speeds up to 1.3Mbit/s using ordinary UART, LIN, SPI and CAN protocols. Optical modem Modems that are used to connect to a fiber optic network are known as optical network units (ONUs). Fiber optic systems can be upgraded by the use of quadrature amplitude modulation. The modulator and demodulator are separate components rather than a single assembly as with most modems. QAM16 16QAM uses a 16-point constellation to send four bits per symbol. Speeds are usually on the order of 200 or 400 gigabits per second. QAM64 64QAM uses a 64-point constellation to send six bits per symbol. Although suppliers have announced components, announcements of installation are rare. Speeds of 65 terabits per second have been observed. Home networking Although the name modem is seldom used in this case, modems are also used for high-speed home networking applications, especially those using existing home wiring. One example is the G.hn standard, developed by ITU-T, which provides a high-speed (up to 1 Gbit/s) local area network using existing home wiring (power lines, phone lines and coaxial cables). G.hn devices use orthogonal frequency-division multiplexing (OFDM) to modulate a digital signal for transmission over the wire. The phrase "null modem" was used to describe attaching a specially wired cable between the serial ports of two personal computers. Basically, the transmit output of one computer was wired to the receive input of the other; this was true for both computers. The same software used with modems (such as Procomm or Minicom) could be used with the null modem connection. Voice modem Voice modems are regular modems that are capable of recording or playing audio over the telephone line. They are used for telephony applications. See voice modem command set for more details on voice modems. This type of modem can be used as an FXO card for private branch exchange (PBX) systems (compare V.92). See also References External links * Hayes-compatible Modems and AT Commands from the Serial Data Communications Programming Wikibook * International Telecommunications Union ITU: Data communication over the telephone network * – no longer available, archived version * Basic handshakes & modulations – V.22, V.22bis, V.32 and V.34 handshakes * Getting connected: a history of modems – techradar * Data/FAX Modem Transmission Modulation Systems – baud rates and modulation schemes Category:Modems Category:Computer-related introductions in 1958 Category:American inventions Category:Bulletin board systems Category:Logical link control Category:Physical layer protocols